Over the years, I've started accumulating some earphone accessories to travel comfortably among airlines of varying prices and different kinds of aircraft.
What would be a full travel kit for earphone accessories, assuming one wants to use the in-flight system?
At some point it will be simpler and cheaper to simply carry your own earphones + tablet, but still, it may be worth putting all of it together (e.g. if the in-flight system has access to movies you don't have in your device).
Here are the accessories I currently carry:
Stereo to mono adapter
Some aircraft have 2 holes for stereo sound, so connecting earphones to only one of them may lead to sound being played to only one of your ears. This adapter will duplicate the sound to both ears (losing the stereo in the process, but at least improving comfort).
Analog in-line volume control (to lower minimum volume)
Sometimes the minimum volume is too loud (especially if your earphone has a difference impedance than the ones given/sold by the airline), so this analog volume control allows fine-grained volume reduction. It cannot amplify the sound, however.